The present invention relates to production of powder metal bodies by means of xe2x80x9cisostaticxe2x80x9d compaction or compression, and in particular symmetrical solids of rotation produced by axial compression of a metal powder aggregate, by means of one or more punches compressing a powder aggregate in a compression chamber provided in a die.
At axial compression of powder aggregates for production of such rotational symmetrical bodies, such as cylinders or the like, it is often a problem that it is difficult to impart upon the entire body a sufficiently high and uniform pressure. There is a tendency that the pressure will be high close to the die but decrease with the distance from the punch. If pressurizing the entire body in a uniform manner is successful, the body is often subjected to stresses, which might often be so big, that the body will be subjected to cracking or scoring when removed from the die. The expelling of the compact furthermore requires very large forces.
An object of the present invention is to provide a method for isostatic compression of powder metal bodies, whereby the above problems are eliminated. According to one aspect, the present invention provides a method for producing a rotational symmetrical powder metal body comprising:
a) introducing a predetermined volume of powder material into a compacting chamber, provided in a hollow die comprising at least one die plate having a central opening, which has substantially the cross sectional shape of the body to be produced,
b) compacting the material in mainly an axial direction in the compacting chamber with a first punch, by urging said first punch axially into said compacting chamber,
c) advancing said first punch further into said compacting chamber, thereby further compacting the powder material,
d) imparting upon the at least one die plate a biasing tension, causing an edge portion encircling the opening in the at least one die plate to move slightly under increasing tension in the direction of the advance movement of the first punch, thereby causing radial reduction of the size of the opening,
e) causing axial compression also in a direction opposed to that caused by the said first punch by means of a second punch, arranged to move relative the die in a direction opposed to that of the advance movement of the first punch, and
f) retracting the first punch axially, and relieving the biasing tension from the at least one die plate thereby regaining its initial form and simultaneously expelling the compacted material.
Another object of the invention is to provide a device for performing the method according to the invention. According to a further aspect, the present invention provides a device for compression of rotational symmetrical powder metal bodies, comprising: a die comprising at least one die plate having a through-opening forming a substantially cylindrical compacting chamber, a first punch movable in a direction substantially parallel to the axis of said through-opening and between a first position exposing the top part of the through-opening and further positions wherein the first punch is positioned inside the through-opening, a second punch provided at an end of the through-opening opposite to the first punch, the at least one die plate, at least at its center portion surrounding the through-opening is elastically movable in a direction substantially coinciding with advance movement of the first punch, the at least one die plate being arranged to be subjected to a biasing force in the advance direction of the first punch, thereby obtaining an increasing biasing and a simultaneous reduction of the radial size of the opening.